North Korea has threatened to attack South Korea if Seoul joins a new round of tightened UN sanctions, as Washington unveils more of its own economic restrictions following Pyongyang’s rocket launch last month.

In a third day of fiery rhetoric, North Korea directed its verbal onslaught at its neighbour on Friday, saying: “‘Sanctions’ mean a war and a declaration of war against us”.

The reclusive country this week declared a boycott of all dialogue aimed at ending its nuclear programme and vowed to conduct more rocket and nuclear tests after the UN security council censured it for a long-range missile launch in December and expanded existing sanctions.

“If the puppet group of traitors takes a direct part in the UN ‘sanctions’, the DPRK [Democratic People's Republic of Korea] will take strong physical counter-measures against it,” North Korea’s committee for the peaceful reunification of Korea said, referring to its neighbour.